The present invention relates to ladders, and more specifically to a combination step/extension ladder which may be arranged in an extension ladder configuration or, with the slightest of ease, can be transformed into a step ladder configuration.
There are a variety of different forms of ladders used in today's industry, these include aerial ladders, step ladders, extension ladders (lean to ladders) and accommodation ladders. Of these, the most commonly used ladders are the step and extension ladders. The extension ladder is basically a "lean to" ladder which is comprised of a multitude of sections which telescopically extend the length of the ladder. This ladder is supported by leaning it up against a structure, such as a wall, with its base firmly planted on the ground. On the other hand, the step ladder is generally a folding type structure which, unlike the extension ladder, is self-supporting (because of its A-frame structuring).
There are a number of ladders which are constructed to combine the principles of the step ladder with those of the extension ladder. See, e.g., U.S. Pat. Nos. 274,526; 830,485; 1,401,257; 1,691,976; 2,899,008; and 4,376,470.
Generally, these combination ladders are constructed in two different manner: one is to have the two sections of the ladder pivot about a hinge at the top of the step, such that when folded out it is used as an extension ladder, and when folded inward, to a basic A-frame, it is used as a step ladder; the second manner of construction maintains the ladder in basically the step ladder construction (A-frame) wherein the rungs are extended upward by sliding the step ladder upward while still keeping the A-frame feature.
There are many drawbacks to these prior designs for combination step/extension ladders. For example, in the construction which pivots about a hinge, the structure is not considered very stable. The stresses imposed on the hinge is very high and this design allows the extended ladder to wobble considerably more at this hinge/flexure point. Also, the previous design do not consider the angle of the rungs on the inverted section, thereby causing an unsafe ascent for the person using the ladder. The rungs should be constructed such that the rungs are constant, that is they do not change in shape or structure. A change in the rung, angle or size is generally not anticipated by one using a ladder, therefore the change in the rung may cause an accident for an unwary user.
Further, problems with the A-frame constructed step/extension ladder are associated with safety as well. One major problem revolves around the need to adjust the bottom width of the A-frame. In extending the ladder height the previous design require that the A-frame be widened by readjusting the spreader strap which hold the legs of the A-frame together. This does not allow for stable construction of the A-frame, because there is an increase in the chances for failure at the spreader strap and constant changes at this point would cause some degree of fatigue to whatever material is used.
It is toward the solution of these inherent problems that the present invention is directed.